refactor: Remove isDart from public API

BREAKING CHANGE:

- `IS_DARTIUM` no longer exported

modules/angular2/src/facade/lang.dart

@@ -5,8 +5,6 @@ import 'dart:math' as math;
 import 'dart:convert' as convert;
 import 'dart:async' show Future;
 
-bool isDart = true;
-
 String getTypeNameForDebugging(Type type) => type.toString();
 
 class Math {

modules/angular2/src/facade/lang.ts

@@ -15,8 +15,6 @@ export function getTypeNameForDebugging(type: Type): string {
   return type['name'];
 }
 
-export var isDart = false;
-
 export class BaseException extends Error {
   stack;
   constructor(public message?: string, private _originalException?, private _originalStack?,